Oscilloscopes are often times utilized for providing a visual representation of an input signal. It is desirable for the oscilloscope to allow viewing of the leading edge of an input signal.
In conventional oscilloscopes in order to allow an operator to view a leading edge of the input signal, a stable pre-trigger signal must be provided. Circuitry that is provided in conventional oscilloscopes for producing the stable pre-trigger signal makes the conventional oscilloscope cumbersome to operate. The circuitry utilized in the conventional oscilloscopes tends to suffer from time jitter and consequent performance degradation as well.
The use of superconducting devices and particularly Josephson tunneling devices, in sampling or Analog/Digital (A/D) circuits is already known in the art. Use of a Josephson device provides a very sensitive detector, offering the possibility of very fast sampling speeds because such a device is capable of extremely fast switching speed between two stable states and because the device responds to extremely small magnetic fields.
U.S. Pat. No. 4,401,900 shows a Josephson sampling technique with a time resolution of 5 picoseconds and a sensitivity of 10 microvolts. The time resolution for the described sampling system is extendable to the subpicosecond domain, limited ultimately by the intrinsic switching speed of the Josephson device used as the sampling gate. In principle, the switching speed can be as little as 0.09 picoseconds.
Josephson sampling techniques are not restricted to only those waveforms produced in a cryogenic environment. Rather, they can be used to measure waveforms from various sources such as x-rays, optical photons or electrical waveforms produced by room temperature sources, if a suitable interface is available. Examples of such interfaces are described in the co-pending patent applications Ser. No. 796,841 entitled "Room Temperature to Cryogenic Electrical Interface" filed on Nov. 12, 1985 and Ser. No. 796,842 entitled "Open Cycle Cooling of Electrical Circuits" filed on Nov. 12, 1985.
The Josephson sampling system described in U.S. Pat. No. 4,401,900 comprises a superconductive monitor gate such as a single Josephson device, which has at least two states distinguishable from one another and which is sensitive to an unknown waveform or signal to be sampled. Switching means which include, the source of the unknown signal, a source of timing pulses and a source of bias signal change the state of a monitor gate by a proper combination of the above signals. A timing means is provided to establish both a timing reference and an accurate sampling delay time. The timing means includes a pulse generator for providing very short sampling pulses, delay lines and a source of trigger pulses. The sampling system also has noise elimination means to insure the accuracy of the sample at any given instant of time and a display to indicate the unknown waveform.
A sampling system that utilizes superconducting devices must be integrated with other circuitry so as to enable the viewing of the leading edge of an input signal in an oscilloscope. Circuitry utilized with the sampling gate in a sampling oscilloscope should provide low time jitter triggering and high performance.